What\'s the Difference Between `map()` and `flatMap()` in Java 8 Streams?

Understanding the Discrepancy Between Java 8's map() and flatMap() Methods

In Java 8, both map() and flatMap() methods can be applied to a Stream, returning a Stream. However, these methods differ in their input-output relationship.

map() Method

The map() operation generates one output value for each input value. Essentially, it applies a Function to each element in the input stream. The Function takes the input value and produces a single result value.

flatMap() Method

In contrast, the flatMap() operation can produce any number of values (including zero) for each input value. This is enabled by its argument: a function that takes a value and returns an arbitrary number of values.

The reason for this difference is that Java methods can only return zero or one value. flatMap() sidesteps this limitation by allowing the mapper function to return a stream as its output. Values from this stream are subsequently drained and passed to the output stream. Consequently, the output is considered "flattened" since the mapper function's value groupings are not distinguishable in the output stream.

Usage

Typically, flatMap() is used when the mapper function intends to return zero values (using Stream.empty()) or multiple values (using Stream.of(a, b, c)). Nevertheless, any type of stream can be returned.

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